SQL Server Recovery Models

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In my last post (A Beginner’s Guide to SQL Server Backups) we discussed the basics of SQL Server Backups.  As Backups are the foundation for Recovery, the next logical discussion should be Recovery Models.  For reference, I have included this Sample Backup Plan from the earlier post:

Sample Backup Plan

Sample Backup Plan

There are Three SQL Server Recovery Models

Simple Recovery Model

This is the most basic of the Recovery models.  It gives you the ability to quickly recover your entire database in the event of a failure; (or if you have the need to restore your database to another server) however, this only recovers data to the end of the last backup.

Thankfully, Differential Backups can be utilized with this recovery model. NOTE: Any changes made after the last Full or Differential Backup will be lost.  Transaction Log Backups cannot be used in Simple Recovery Model.  There is no Point-In-Time recovery in this model (recovering to a specific transaction or point in time).  Looking at the sample backup plan above… if you have an issue occur Saturday afternoon, you will lose any changes in data since your last Differential Backup that ran Friday night.  If you do not opt for Differential Backups and only perform Full Backups once a week, your changes and data will be lost for the full week.  (GASP!)

Why would you ever consider using such a basic, Simple Recovery Model?  Actually, there are a few really good (and perfectly safe) reasons why you choose to use Simple Recovery Model:

  • Your data is not critical and can easily be recreated
  • The database is only used for test or development
  • Data is static and does not change
  • Losing any or all transactions since the last backup is not a problem

Full Recovery Model

This is the most inclusive of the Recovery models; you can recover (depending on your valid backups) data up to the last transaction that was run before the failure. Log backups are a must for this recovery model, including Tail Log Backups (which will be discussed at length in a later post).  Data can be restored to a specific point in time (once again, depending on your backup plan)!  

        • Let’s explore that for a moment using the sample backup plan above.  Say you take Full Backups on Sunday night, Differential Backups every week night, and Transaction Log Backups every half hour.  If you have a failure and must restore data, by using all the backups to the specific time you need to restore, you can recover data at any moment in time!  Using this recovery model, you have a potential of minimal to no data loss (once again…depending on your backup plan and whether your backups are restorable).  You want to be the DBA who is able to restore quickly and recover data to the very last moment before the failure or crash occurred.  YES, we all want to be THAT person!

Bulk Logged Recovery Model

Once again, this Recovery model requires log backups in order to prevent the log file from continually growing.  This is a special purpose recovery model that is not widely used, and Microsoft states that it should only be used “intermittently to improve the performance of certain large-scale bulk operations, such as bulk imports of large amounts of data.”   It allows high performance bulk-copy operations and is only available when in Full Recovery.  Keep in mind that with the Bulk Logged model you can recover to the end of any backup; however, if a bulk transaction has occurred during the last log backup, those transactions will have to be redone.  From the bulk operation on, you can no longer utilize point in time restore. 

OOOOOOOOOO, I dropped the ENTIRE database!

While studying Backups and Recovery, I have been working in SQL Server 2016 AdventureWorks Database (Download Here).  To see if I REALLY understood the process of Backup and Recovery, I made a Copy Only Backup of my original database, made some changes to my database, took Differential Backups, made another Full Backup, made more changes, and made more backups….and then dropped EVERYTHING.  POOF….gone.  While I did do this intentionally, my heart was racing.  Sparkly spots darted in and out of my vision, and my heart pounded in my ears.  Would I be able to get it back?  Would I be able to restore it?  WHAT HAVE I DONE??????

Photo Credit @Lance_LT

I Dropped the DB! Photo Credit @Lance_LT

How do I know that my backups were good?

I firmly believe you learn more from one of your failures than you do from one hundred of your successes.  TEST TEST TEST TEST those backups!  I am afforded some extra wiggle room while I am working on my virtual machine and not actually working on a client’s production database.

So, I did it, I dropped that DB like a hot rock.  I attempted to restore my database in Simple Recovery Model.   My Differentials would not restore.  Somewhere along the way in the backup or restore process, I messed up.   (I will give you a hint at my mistake, I did not restore the Full Backup with NORECOVERY before I tried to restore the Differential Backup with RECOVERY – we will go into Recovery processes in a later post).  

Knowing that I had made a Copy Only Backup before I started monkeying around with the database, I set out to restore faith in myself.  And boom, I restored the Copy Only Backup!  The day was saved!  Well, mostly.  I lost some data changes, but still had the original database, so that was good-ish.

Lessons Learned

At a later date I repeated the steps above and was able to fully recover a database that was previously in Simple Recovery Model complete to the last Differential Backup (noting my mistake and learning from it).  I additionally created another database to test the Full Recovery Model with Full Database Backup, Differential, Transaction Log Backups, also utilizing Tail of the Log backup to get to the very last transaction that was run after the last backup and before my data files AND database were deleted. (This is pretty in-depth and is covered in Recovery Using Tail Log Backup.)

Please come back next time when we will explore ways to validate those backups.

Thank you for reading!

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A Beginner’s Guide to SQL Server Backups

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As was discussed in a previous blog, (The Most Important Role of a SQL Server DBA) Backups and Recovery are the cornerstone of all successful SQL Server DBA careers.  Exactly how much attention is paid to backups until something drastic happens?  NOT AS MUCH AS SHOULD BE!

What is a SQL Server Backup?

A backup is the process of putting data into backup devices that the system creates and maintains.  A backup device can be a disk, file, a tape, or even the cloud.  Easy enough!

Backups are your Keys to Success

Backups are your Keys to Success

There are four different methods of backup in SQL Server:

  • Full Backup – A Full Backup is a copy of the entire database as it exists at the time the backup was completed. It is all-encompassing; every detail of your database (data, stored procedures, tables, functions, views, indexes, and so forth) will be backed up. This is why the Full Backup is the foundation of every restore sequence.  These Full backups can be quite large and put a strain on your storage capacity.  Despite the name, a full backup will only backup the data files; the transaction log must be backed up separately.  To ensure you can recover to the point of failure quickly, you will want to also utilize Differential and Transaction Log Backups (we will cover these next).  Without a Full Backup, your Differential and Transaction Log Backups are useless as you cannot restore a database without a Full Backup!  
  • Differential Backup – Differential Backups capture only changed data from the last Full Backup. Simple terms: this will backup only the changes since the last Full backup, not backup the entire database. In the event of an outage, the Differential Backups can greatly reduce the time to recover.  Using both Differential Backups and Full Backups will dramatically reduce required storage space, as the Full Backups can be very large and the Differentials are remarkably smaller.  
    • How does a Differential Backup know what data has changed?  When data is changed on a page, it sets a flag.  This flag indicates which extent (collection of 8 database pages) the Differential Backup will need to backup.  If your Differential Backups are set to run daily, each day’s data changes are recorded and the flag will not be cleared until a Full Backup has been executed.  Say on Monday you have 3 flags set, and Thursday you have 4 more set, your differential backup on Thursday night will contain data changes represented by all 7 flags, so the flags from the Monday Differential are not cleared by the subsequent Differential Backups.  The flags would only be cleared by a Full Backup.
  • Transaction Log Backup – A Transaction Log is a physical file that stores a record of all the transactions that have occurred on a specific database; much like a recorder, maintaining a record of all the modifications.  The Transaction Log Backup will capture the information and then releases the space in the log file so it is able to be used again.  In doing this, the Transaction Log Backup truncates the information in the Transaction Log, but the data is not deleted, it is stored in the backup!  Without Transaction Log Backups, the Transaction Logs will run and grow nonstop, chewing through valuable storage space.   
    • Not only does backing up and truncating the Transaction Log manage the filesize, it also decreases the potential for catastrophic data loss.  Having all the Transaction Log Backups since your last Full Backup will allow you to perform point in time restores.  Transaction Log Backups are ideally set up to execute every few minutes to every hour, depending on your company’s threshold for data loss.
  • Copy Only Backup – A Copy Only Backup doesn’t change the Differential.  It can be a full backup in that it is a record of the database as it exists when the backup is taken; however, it does not reset flags.  It is ideal for doing a full backup in the middle of the week without disrupting any other backups.  A Copy Only Backup can be used in a Dev space for troubleshooting, or preparing to move to a new environment.

Here is an example of a solid weekly backup plan which uses Full, Differential, and Transaction Log Backups:

Sample Backup Plan

Sample Backup Plan

Okay, great…backed up, now what?

Okay, so now you know what SQL Server backups are, a description of each backup type, an idea of how and what they back up, and have an idea of a good plan of action to create a solid backup plan.  So, why are backups so important?  Do you know how easy it is to accidentally update or delete data?   It is just one T-SQL statement away with the wrong filter, or no filter at all.  Having good up to date backups and being able to restore them is the difference between looking like a hero and being forced to find a new job.  Backups are your foundation for Recovery.

Please come back, this is the 2nd in a series of blog posts regarding SQL Server Backups and Recovery.  See you next time when we begin to discuss SQL Server Recovery Models.  Thank you for reading!

 

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